Migrating the Data.concatenate_data method towards #182, by deprecating it.

I don't think this approach will be controversial (so-as to have required pre-PR discussion) since it seems to be essentially the same method as concatenate, just compare the inputs and outputs on the docstrings to see this, with no real difference except for the behaviour of:

In the case that the list contains only one element, that element is simply returned.

and it is not used anywhere throughout the codebase, unlike concatenate which is used quite widely (note this was a post-PR grep, but regardless the only usage is to define it):

$ pwd
/home/sadie/cf-python/cf
$ git grep "concatenate_data"
data/mixin/deprecations.py:    def concatenate_data(cls, data_list, axis):
data/mixin/deprecations.py:            "Data method 'concatenate_data' has been deprecated at "

Anyhow @davidhassell you can object at this stage and close if there is a justification to keep the method, else review this towards merging, as the change itself is almost trivial. Thanks.